Speed control of a wye-connected induction motor utilizing delta connected triggerable biconductive devices

ABSTRACT

A variable speed polyphase induction motor system features phase-controlled thyristors connected in full-wave delta in the neutral of a wye-connected motor.

United States Patent inventors Appl. No. Filed Patented AssigneeHerbert]. Brown Scotia;

George M. Rosenberry, Jr.. Schenectady,

both of, N.Y. 866,103

Oct. 7, 1969 June 1, 1971 General Electric Company Continuation ofapplication Ser. No. 620,123, Mar. 2, 1967. now abandoned. Thisapplication Oct. 7, 1969, Ser. No.

Primary Examiner-Gris L. Rader Assistant Examiner-Gene Z. RubinsonAttorneys-James C. Davis, Jr., Lawrence G. Norris, Melvin M. Goldenberg,Frank L. Neuhauser and Oscar B. Waddell DEVICES 3 3 Drawing ABSTRACT: Avariable speed poiyphase induction motor US. Cl. 318/227, systemfeatures phase-controlled thyristors connected in full 318/230 wavedelta in the neutral of a Wye-connected motor.

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I scRa CURRENT IN AHPERES TIME mo I Um; I SCR66 INVENTORS HERBERT J.BROWN GEORGE r1. ROSENBERRY, JR.

THEIR ATTORNEY SPEED CONTROL OF A WYE-CONNECTED INDUCTION MOTORUTILIZING DELTA CONNECTED TRIGGERABLE BICONDUCTIVE DEVICES I 2 This 3 is4 a 5 continuation 6 of 7 application 8 Ser. 9 No. 5

I0 11 620,123 l2 l3 filed 14 Mar. 15 162, 17 l967 l8 l9 and 20 now 21abandoned.

BACKGROUND Numerous combinations of phase-controlled thyristors inseries with polyphase loads, including the windings of induction motors,have been proposed. These include: (I) the use of a thyristor in serieswith each of the three power leads to deliver controlled half-wavepower, (2) the use of diodes to permit unrestricted bypassing of theaforementioned thyristors in the reverse direction to provide full-wavepower with half-wave control, and (3) the use of additional thyristorsto replace the aforementioned diodes to gain full-wave power andcontrol. Various combinations of the foregoing in less than all threepower leads also have been proposed.

In general, it has been found that half-wave power and halfwavecontrolled systems result in substantial extra motor losses, andunbalanced control systems produce a correspondingly undesirableunbalanced line and load condition and extra losses. Of the systemsdescribed, the full-wave power and full-wave control system representsthe most promising one; however, there is need for a system of this kindwherein the current capacity of the thyristors is more effectively andfully utilized, in the interest of controlling motors of greater powerwith thyristors of a given rating.

OBJECTS It is a principal object of this invention to provide animproved adjustable speed polyphase induction motor.

It is a further object of this invention to provide improved means forcontrolling the quantity of electric power supplied from a three-phasealternating-current source to a three-phase winding load.

It is another object to provide a polyphase adjustable speed inductionmotor utilizing phase-controlled thyristors of lesser current ratingthan would be required in systems heretofore known with a motor ofsimilar rating.

BRIEF DESCRIPTION In a preferred embodiment of the present invention, atriac, or an equivalent pair of back-to-back SCRs, is connected in eachof the three legs of a delta and the neutral ends of the three phasewindings of a Wye-connected motor are respectively connected to thethree points of the delta. Each of the triacs, or pairs of SCRs, istriggered symmetrically in respect to the line voltage applied to thetwo windings, the neutrals of which they connect. By varying the timedelay, or phase angle, of triggering, the power input to the motor iscorrespondingly varied to provide torque or speed control. It has beenfound that this system permits smaller conduction angles in thesolidstate devices and substantially reduced average current ratingsthereof when compared with a full-wave system utilizing devicesconnected in series with the power lines and providing the same controlfor a given motor.

DRAWING FIG. 1 is a schematic circuit diagram of a variable speedpolyphase motor system in accord with the present invention;

FIG. 2 is a schematic circuit diagram of a time-delay pulse generatorsuitable for use in the system of FIG. 1;. and

FIG. 3 is a plot of current versus time illustrating the relationshipbetween the magnitude of line current and phase current in the system ofFIG. 1.

DETAILED DESCRIPTION The dashed-line rectangle 10 in FIG. I encloses aschematic representation of a dynamoelectric machine primary windingthat in a typical case can be the stator winding of a conventionalpolyphase induction motor, for example. The primary winding illustratedcomprises three phase windings 12, 14 and I6 that, in turn, can besubdivided into a plurality of series and/or parallel-connectedcomponents (not shown), all as is well known in the art. Phase windings12, I4, and 16 include respective terminals, or line connection leads,I8, 20 and 22 at one extremity thereof and respective neutral terminals,or leads, 24, 26 and 28 at the other extremity thereof. Externalaccessibility in a junction box orthe like of both ends of phasewindings is frequently provided in this way in dynamoelectric machines,particularly in motors of greater than fractional horsepower, in orderto facilitate manual or automatic reconnection of the winding fordifferent magnitude voltages and/or for motor starting with reducedmagnitude initial current surges, for examples.

Line terminals 30, 32 and 34 are respectively connected to connectingleads I8, 20 and 22, as by respective conductors 36, 38 and 40, to adaptthe primary winding to be connected to a conventional source ofpolyphase alternating-current power. In the illustration, such a sourceis represented by power bus terminals 42, 44 and 46, which are connectedrespectively to terminals 30, 32, and 34 by respective conductors 48, 50and S2. The power line connection is only made directly to one end ofwindings 12, I4 and 16, in which case the dynamoelectric machine isreferred to as Wye-connected, or Y-connected, because of the orientationof the windings to each other in the usual schematic representationsthereof. I

In accord with the present invention, three bidirectionally conductive,triggerable semiconductive means 54, 56 and 58 are connected together ina closed-loop delta having three junctions, or delta points, designatedat 60, 62 and 64. semiconductive means that can be triggered, or fired,into the highly conductive state from a relatively nonconductive statein either polarity of conduction are available in a single device, asthe well-known triac, for example, or each semiconductive means cancomprise more than one device, as shown, in the form of two SCRs or thelike connected in inverseparallel, or back-to-back, relationship. In theillustrative embodiment, semiconductive means 54 includes SCR 66 and SCR68 with the principal electrodes thereof connected in inverse-parallelrelationship, i.e., the anode of SCR 66 and the cathode of SCR 68 areboth connected to junction 64, and the cathode of SCR 66 and the anodeof SCR 68 are both connected' to junction 60. Similarly, SCRs 70 and 72comprise semiconductive means 56 and SCRs 74 and 76 comprisesemiconductive means 58, to form the other two sides of the delta.

Means are provided respectively connecting the three neutral terminals24, 26 and 2 8 to the delta junctions 60, 62 and 64. In FIG. 1, suchmeans are represented by conductors 78, 80 and 82, respectively. Thus,the neutral terminals 24, 26 and 28 are not connected together as in aconventional wyeconnected machine, but are selectively electricallyjoined in pairs only by conduction of one or more of semiconductivemeans 54, 56 and 58.

In order to selectively establish conduction in the aforementionedsemiconductive means, trigger means are provided coupled thereto. InFIG. 1, the trigger means comprise timedelay pulse generators 84, 86 and88 which serve semiconductive means 54, S6 and 58, respectively. Towardthis end, each of the pulse generators 84, 86' and 88 are illustratedschematically to be connected in series with a pulse transformerprimary, designated at 90, 92 and 94, respectively, across theparticular power bus terminals that supply the power to the windings,the neutrals of which, are joined by their correspondingly controlledsemiconductive means. Coupling to the semiconductive means is effected,in the case of pulse generator 84, by secondary windings 96 and 98 oftransformer 90. Winding 96 is connected from the cathode to the gate 100of SCR' 66 and winding 98 is coupled from the cathode to the gate 102 ofSCR 68. Similar coupling of transformer primaries 9'2 and 94 is alsoeffected, but is not shown to avoid congestion in the drawing. In manycases, the duration of timedelay is advantageously controlled inresponse to the magnitude of an electric control voltage or current, byany of a plurality of wellknown means, and control signal inputterminals I04, 106 and 108 schematically illustrate means adapted forsuch connection in pulse generators 84, 86 and 88, respectively.

FIG. 2 illustrates a suitable time-delay pulse generator and comprises acurrent-limiting resistance H0, a variable resistance 112 (that couldrepresent a control transistor, for example) and a capacitance [14connected in series circuit together and in series with transformerprimary 90 from bus 48 to bus 50. A triac 116 is connected in parallelwith resistance 112 and capacitance 114 and the triac gate is connectedto the junction of the latter two components by means of diac 117, orsilicon bilateral switch, for example. Alternating positive and negativepulses are provided in primary 90 after capacitance 114 charges in thecorresponding polarity to the breakover voltage of diac 116. The timerequired, after a voltage zero between lines 48 and 50, for the requiredcharging to occur depends upon setting of resistance 112.

Many other firing circuits are known which are suitable for use inconnection with this invention or can be readily adapted for such use.Of course, it is not required that the pulse transformer primaries beconnected in series with the pulse generators, as illustrated,Alternating polarity pulses are not required because the semiconductivemeans can only become conductive in one direction at a time, dependingupon the polarity of voltage thereacross at the time a trigger pulse isreceived. For other firing circuits, specific reference is hereby madeto the SCR Manual," third edition, published in 1964 by the assigneehereof, particularly the section commencing at page 130 therein entitlesTrigger Circuits for Phase Control." Unijunction transistor firingcircuits are particularly useful, and many electronically variabletime-delay, or phase shift, means therefor are well known,

FlG. 3 is a copy of an oscillogram taken from a system in accord withthe invention and illustrating the magnitude of phase current and linecurrent on the ordinate and time on the abscissa for one complete cycle.The motor was a three-phase, 50 horsepower induction machine turning at1200 with a fan load of 86 ft.-lb. torque. With the motor-Wye, SCR-deltasystem herein described, the total conduction angle of each SC R was 78and the average current was 17.0 amperes. Under the same conditions ofmotor operation, a full-wave system with SCRs in series in the lineshowed a conduction angle for each SCR of 1 17 and an average current of32.3 amperes. Thus, the average SCR current in accord with thisinvention is 53 percent of the value obtained with the usual inlineconnection. The peak current is approximately equal in each case, but inthe system of this invention two SCRs, rather than one SCR, carry themotor line current each half cycle. Tests have shown that the requiredSCR voltage rating is the same for both cases. This means that a givenset of SCRs can handle a motor of nearly twice as great horsepower inaccord with the present invention.

Under typical operating conditions, as described above, in a full-wavesystem wherein the motor windings and SCRs were both in deltaconfiguration, the stator losses were found to be 134 percent greaterthan in the motor-wye, SCR-delta system of this invention. Thecorresponding losses were 149 percent greater in the case of a half-wavesystem using three SCRs in a closed-loop delta.

While the invention is described in the illustrative setting of variablepower polyphase induction motors, it will be apparent that the teachingherein extends to the general environment of controlling the quantity ofpolyphase power applied to a polyphase load having neutral leads,particularly in the case of polyphase transformer windings.

The foregoing is a description of an illustrative embodiment oftheinvention, and it is applicants intention in the appended claims tocover all forms which fall within the scope of the invention.

We claim:

1. A dynamoelectric machine comprising:

a. a polyphase winding having three line terminals and three neutralleads;

b. three bidirectionally conductive and triggerable semiconductive meansconnected together in a closed-loop delta having thrcejunctions;

c. means respectively connecting said three neutral leads to saidthreejunctions;

d. means for connecting a three-phase power supply to said three lineterminals; and

e. trigger means coupled to said semiconductive means for selectivelyestablishing conduction therein, said trigger means providing conductioninitiating trigger pulses to predetermined ones of said bidirectionallyconductive semiconductive means during the positive and negative halfcycles of the three-phase energization waveform applied to eachpolyphase winding to produce current flow in opposite directions throughsequential legs of said delta connected semiconductive means duringalternate half cycles of sequential phases in said three-phase powersupply and means for varying the phase relationship of said triggerpulses relative to the voltage at said line terminals to correspondinglyvary the quantity of power supplied to said windings.

2. The machine of claim 1 wherein said trigger means com prises avariable time-delay pulse generator coupled to each of saidsemiconductive means and means synchronizing operation of said pulsegenerators with the voltage applied to the line terminals of thetwo-phase windings whose neutral leads are connectable by conduction ofthe respective semiconductive means to which said generators arecoupled.

3. A variable speed polyphase induction motor comprising:

a. a stator having first, second and third phase windings therein, eachwinding have two extremities;

b. first bidirectionally conductive triggerable semiconductive meansconnected to one extremity of said first phase winding and to oneextremity of said second phase winding, second bidirectionallyconductive triggerable semiconductive means connected to said oneextremity of said second phase winding and to one extremity of saidthird phase winding, and third bidirectionally conductive triggerablesemiconductive means connected to said one extremity of said third phasewinding and to said one extremity of said first phase winding, each ofsaid semiconductive means being triggerable to establish the only pathof current conduction between the winding extremities to which it isconnected;

c. means for energizing the second extremity of each of said windingswith an individual phase of a three-phase power source,

d. trigger means coupled to said semiconductive means for selectivelyestablishing conduction in opposite directions in the semiconductivemeans connected to the opposite extremity of a phase winding duringsuccessive half cycles of the power source phase connected to the secondextremity of the phase winding, said trigger means comprising a firsttrigger pulse generator coupled to said first semiconductive means andconnected to the other extremities of said first and second phasewindings, a second trigger pulse generator coupled to said secondsemiconductive means and connected to said other extremity of saidsecond phase winding and to the other extremity of said third phasewinding, and a third trigger pulse generator coupled to said thirdsemiconductive means and connected to said other extremity of said firstphase winding, each of said trigger pulse generators having a variabletime delay to alter the phase relationship of said generated pulsesrelative to the voltage applied between the other extremities to whichsaid pulse generators are connected.

1. A dynamoelectric machine comprising: a. a polyphase winding havingthree line terminals and three neutral leads; b. three bidirectionallyconductive and triggerable semiconductive means connected together in aclosed-loop delta having three junctions; c. means respectivelyconnecting said three neutral leads to said three junctions; d. meansfor connecting a three-phase power supply to said three line terminals;and e. trigger means coupled to said semiconductive means forselectively establishing conduction therein, said trigger meansproviding conduction initiating trigger pulses to predetermined ones ofsaid bidirectionally conductive semiconductive means during the positiveand negative half cycles of the three-phase energization waveformapplied to each polyphase winding to produce current flow in oppositedirections through sequential legs of said delta connectedsemiconductive means during alternate half cycles of sequential phasesin said three-phase power supply and means for varying the phaserelationship of said trigger pulses relative to the voltage at said lineterminals to correspondingly vary the quantity of power supplied to saidwindings.
 2. The machine of claim 1 wherein said trigger means comprisesa variable time-delay pulse generator coupled to each of saidsemiconductive means and means synchronizing operation of said pulsegenerators with the voltage applied to the line terminals of thetwo-phase windings whose neutral leads are connectable by conduction ofthe respective semiconductive means to which said generators arecoupled.
 3. A variable speed polyphase induction motor comprising: a. astator having first, second and third phase windings therein, eachwinding have two extremities; b. first bidirectionally conductivetriggerable semiconductive means connected to one extremity of saidfirst phase winding and to one extremity of said second phase winding,second bidirectionally conductive triggerable semiconductive meansconnected to said one extremity of said second phase winding and to oneextremity of said third phase winding, and third bidirectionallyconductive triggerable semiconductive means connected to said oneextremity of said third phase winding and to said one extremity of saidfirst phase winding, each of said semiconductive means being triggerableto establish the only path of current conduction between the windingextremities to which it is connected; c. means for energizing the secondextremity of each of said windings with an individual phase of athree-phase power source, d. trigger means coupled to saidsemiconductive means for selectively establishing conduction in oppositedirections in the semiconductive means connected to the oppositeextremity of a phase winding during successive half cycles of the powersource phase connected to the second extremity of the phase winding,said trigger Means comprising a first trigger pulse generator coupled tosaid first semiconductive means and connected to the other extremitiesof said first and second phase windings, a second trigger pulsegenerator coupled to said second semiconductive means and connected tosaid other extremity of said second phase winding and to the otherextremity of said third phase winding, and a third trigger pulsegenerator coupled to said third semiconductive means and connected tosaid other extremity of said first phase winding, each of said triggerpulse generators having a variable time delay to alter the phaserelationship of said generated pulses relative to the voltage appliedbetween the other extremities to which said pulse generators areconnected.